The present invention relates to a device and method for monitoring wheel speeds.
In vehicle safety systems, for example in an electronic stability program (ESP), the rotational speed sensor signals may be elementary input variables. The wheel speeds may be calculated from the rotational speed sensor signals with the aid of additional sensor signals. With the aid of the additional sensor signals, the wheel speeds may be related to the vehicle center of gravity and/or the center of the rear axle. These wheel speeds related to the vehicle center of gravity and/or the center of the rear axle may be exactly monitored for numerous functions, for example, for the ABS function (antilock braking system) or the TCS function (traction control system). It may be required, in particular for the TCS part of the ESP, to monitor the wheel speeds very precisely, since narrow regulation thresholds may be used for these functions.
It may be desirable to recognize faulty signals which, for example, may occur due to incorrect or defective gears or even due to rotational speed sensors falling out or too large a clearance.
According to the related art, the slip of the wheels may be monitored on the basis of a logic which is active above the vehicle reference speed of 18 km/h. At 20 km/h, the difference between the slowest and the fastest wheel speeds may not be greater than 2 km/h. This value may correspond to 10%. At 100 km/h, the difference may not be greater than 5 km/h. This may value correspond to 5%. The permissible differences between 18 km/h and 100 km/h may be linearly extrapolated or interpolated as appropriate. Above 100 km/h, the slip differential between one wheel and the other three wheels may not be greater than 5%.
If the difference is greater than that indicated by the limiting values above, an error counter may be incremented. After a specific time period, for example after 20 seconds, an error may be recognized.
In the conventional logic it may be problematic that monitoring is performed solely on the basis of the slowest and the fastest wheel speeds. Particularly at driving speeds below 100 km/h, robustness problems may therefore occur on uneven roadways or, for example, in deep snow.
The present invention may be based on an example method that may provide that, if the differential speed is greater than a threshold value, the wheel having the maximum wheel speed is registered as the fastest wheel and the wheel having the minimum wheel speed is registered as the slowest wheel, and in the event of a change in the wheel speed, so that another wheel is the fastest wheel and/or another wheel is the slowest wheel, the fastest wheel is registered and the slowest wheel is registered and a logical status is determined from the registered values and the instantaneous values. This wheel logic may increase the robustness of the monitoring, since monitoring may not be performed solely by the criterion of the slowest and the fastest wheel speed. Rather, it may be registered whether a change of the slowest and/or the fastest wheel occurs. On the basis of the different cases which may occur, a corresponding status may be determined for the logical combinations occurring. On the basis of such a status, a probable number of faulty wheels may then be deduced.
Furthermore, an error counter may be incremented or decremented, respectively, depending on whether and what type of error has occurred.
Status 0 may be recognized if the slowest wheel and/or the fastest wheel have changed more than once. If the slowest wheel and/or the fastest wheel change more than once during monitoring, an improbability may exist, as may occur on a special road section having uneven roadway (xe2x80x9cwashboardxe2x80x9d). However, an actual error may not be assumed for a situation which corresponds to status 0.
Status 1 may be recognized if only the slowest wheel has not changed or if only the fastest wheel has not changed. Therefore, if the slowest or the fastest wheel was always the same wheel up to this point, this may indicate one single faulty wheel.
Status 2 may be recognized if the slowest wheel has changed exactly once and the fastest wheel has changed exactly once. In the event of status 2, two faulty wheels may therefore probably exist.
Furthermore, status 3 may be recognized if the slowest wheel has not changed and the fastest wheel has not changed. In such a situation, one or two faulty wheels are assumed.
An error counter may be decremented for status 0. Since an error may not be assumed at status 0xe2x80x94it may be a coincidence if an error simultaneously existed upon recognizing status 0xe2x80x94an error counter is decremented. If the error counter is reset to 0 due to this decrementing of the error counter, the registered wheels may be erased. Subsequently, the wheel logic may be reinitialized.
An error counter may be incremented by 2 for status 1 and status 3. After a specific time period, for example after 20 seconds, an error may therefore be recognized.
An error counter may be incremented by 1 for status 2. For status 2, an error may therefore be recognized after, for example, 40 seconds.
It may be useful that the wheel speeds are determined from rotational speed sensor signals and additional sensor signals. Through the combination of the signals the wheel speeds may be determined in relation to the vehicle center of gravity and/or the center of the rear axle, which may be particularly well suited for monitoring.
According to the example method of the present invention, the differential speed may be compared to a relative threshold value. For example, the relative threshold value may be 5% over the entire speed range between 20 km/h and 100 km/h.
However, below a speed threshold, the differential speed may be compared to an absolute threshold value. The speed threshold, below which a constant value is used, may be, for example, 40 km/h, a differential speed threshold of, for example, 2 km/h between the slowest and the fastest wheel being the criterion for initiating monitoring with the aid of the wheel logic according to the present invention.
The present invention may be based on an example device that may provide that, if the differential speed is greater than a threshold value, the wheel having the maximum wheel speed is. registered as the fastest wheel and the wheel having the minimum wheel speed is registered as the slowest wheel, and in the event of a change in the wheel speed, so that another wheel is the fastest wheel and/or another wheel is the slowest wheel, the fastest wheel is registered and/or the slowest wheel is registered and a logical status is determined from the registered values and the instantaneous values. Using this example device, an example method according to the present invention may be implemented.
The present invention may be based on the recognition that by introducing a wheel logic, monitoring having improved robustness may be made available. This may particularly relate to the monitoring of traction control on special road sections (xe2x80x9cwashboardxe2x80x9d) or in the event of other special roadway conditions, for example in deep snow.